Scintillation detectors or direct-converting detectors based on semiconductor materials are generally employed for detecting ionizing radiation, in particular gamma and X-ray radiation. In the scintillation detectors the incident radiation is detected indirectly by way of the excitation of electrons and their conversion into photons. The direct-converting detectors are different therefrom by virtue of their ability to count individual photons. By means of a special semiconductor material the direct-converting detectors are able to count individual photons and consequently can directly identify the radiation.
Existing direct radiation converters are based on semiconductor materials such as CdTe, CdZnTe, CdZnSe and CdZnTeSe, for example. However, with said semiconductor materials, in particular when they are subjected to X-ray and/or gamma irradiation at high flux densities, as are typical or necessary for example in CT equipment, a space-charge region is formed in the direct converter due to slow holes or stationary charges which are usually bound to deep or what are called intrinsic impurities. Said deep impurities (having a depth of up to half the bandgap energy) can capture the charge carriers generated through radiation and recombine with them. The space-charge region generated as a result as well as the reduction in the mobility of all the charge carriers cause a reduction in the externally applied electric field and consequently a reduction in the pulse height, with the result that a considerably lower intensity of radiation is suggested. In other words the spectrum is energetically shifted toward lower values. This effect is called polarization and it limits the maximum detectable flux of a direct-converting detector.
No method is known in the prior art for avoiding polarization under applicable ambient conditions in direct converters without additional structural measures. For example, the use of direct converters having a high hole mobility, such as germanium, is limited due to the low bandgap energy and the leakage current associated therewith. Moreover, germanium cannot be used as a direct radiation converter at room temperature, which means that detector systems of said type need to be cooled to well below 0° C. during application.
In another method an attempt is made to prevent the polarization by applying an external electric or radiation field. The aim of this method is to saturate the deep impurities and not to reduce the hole mobility. This is often carried out prior to the measurement, in which case an additional investment must be made in terms of procedural mechanisms and equipment.